Mass flowmeters for flowing media, which operate according to the Coriolis principle, are known in different embodiments (cf e.g. German Patent Specification 41 24 295 and German Offenlegungsschrift 41 43 361, and the publication cited there in each case at column 1, lines 20 to 27, German Patent Specification 42 24 397 and the publications cited there at column 1, lines 23 to 30, as well as German Offenlegungsschrift 196 01 342) and are increasingly used in practice.
In the case of mass flowmeters for flowing media which operate according to the Coriolis principle, basically one distinguishes between, on the one hand, those whose Coriolis measuring tube is made at least essentially straight, as a rule exactly straight, and, on the other hand, those whose Coriolis measuring tube is made loop-shaped. In addition, in the case of the mass flowmeters under discussion, one distinguishes between, on the one hand, those which have only one Coriolis measuring tube and, on the other hand, those which have two Coriolis measuring tubes. In the case of the embodiments with two Coriolis measuring tubes, they can be hydraulically in series or parallel to one another.
Mass flowmeters of the type in question, in the case of which the Coriolis measuring tube is, or Coriolis measuring tubes are, made straight, with respect to their mechanical construction simple and can consequently be produced at relatively low cost. In this case also it is possible to finish or polish the inner surfaces of the Coriolis measuring tube or Coriolis measuring tubes well. In addition, they have a relatively low pressure loss. In the case of mass flowmeters which operate according to the Coriolis principle, and in the case of which the Coriolis measuring tube is, or the Coriolis measuring tubes are, made straight, it can be disadvantageous that both thermally caused expansions, or thermally caused stresses, as well as forces and moments acting from outside, can lead to measurement errors and to mechanical damage, namely stress cracks.
The technical world has already dealt with the above-mentioned problems in mass flowmeters with straight Coriolis measuring tubes (cf in particular German Patent Specification 41 24 295, German Offenlegungsschrift 41 43 361, and German Patent Specification 42 24 379). In this case, these problems have largely been solved, on the one hand, by having the Coriolis measuring tube and the bridge connected to one another in a way which excludes axial relative motions, so that the axial separation of the Coriolis measuring tube/bridge connecting points represents the oscillation length of the Coriolis measuring tube and, on the other, by locating the Coriolis measuring tube within the bridge with initial tensile stress (German Patent Specification 41 24 295), and/or having the Coriolis measuring tube and the bridge consist of materials with identical or almost identical coefficients of thermal expansion (German Offenlegungsschrift 41 43 361) and/or providing a length variation sensor determining changes in the oscillation length of the Coriolis measuring tube--for the oscillation length and stress-dependent correction of the measurement value--(German Patent Specification 42 24 379). Altogether, a mass flowmeter operating according to the Coriolis principle, with a straight Coriolis measuring tube, which has an error of measurement of only about 0.1% (cf. the prospectus "Zulassung des Corimass G-Gerates zum eichpflichtigen Verkehr" of KROHNE Me.beta.technik GmbH & Co. KG), was made successfully.
Mass flowmeters operating according to the Coriolis principle, which have only one straight Coriolis measuring tube, have considerable advantages as compared with those mass flowmeters which have either two straight Coriolis measuring tubes or one loop-shaped Coriolis measuring tube. The advantage as compared with mass flowmeters with two straight Coriolis measuring tubes in particular is to be seen in the fact that flow separators or flow combiners, which are required in the case of mass flowmeters with two Coriolis measuring tubes, are not needed. The advantage as compared with flowmeters with one loop-shaped Coriolis measuring tube, or with two loop-shaped Coriolis measuring tubes, in particular, is to be seen in the fact that a straight Coriolis measuring tube is easier to produce than a loop-shaped Coriolis measuring tube, that the pressure drop in the case of a straight Coriolis measuring tube is less than in the case of a loop-shaped Coriolis measuring tube, and that a straight Coriolis measuring tube can be cleaned better than a loop-shaped Coriolis measuring tube.
However, mass flowmeters which operate according to the Coriolis principle and have one straight Coriolis measuring tube, also have a physically or mechanically specified advantage (cf. European Offenlegungsschrift 0 521 439).
Mass flowmeters operating according to the Coriolis principle require that the Coriolis measuring tube, or the Coriolis measuring tubes, be put into oscillation by means of at least one oscillation generator; the Coriolis forces, or the Coriolis oscillations, do indeed result from the fact that the Coriolis measuring tube oscillates, or the Coriolis measuring tubes oscillate, and from the flowing of mass through the Coriolis measuring tube, or through the Coriolis measuring tubes.
In the case of mass flowmeters with two straight Coriolis measuring tubes, or with one loop-shaped Coriolis measuring tube or with two loop-shaped Coriolis measuring tubes, the Coriolis measuring tubes, or the parts of the loop-shaped Coriolis measuring tubes, causing oscillation are designed identically and, as a rule, located and excited into oscillation so that they oscillate opposite one another. This has the positive consequence that the oscillating system as a whole is not acting as such outwards. The position of the center of mass remains constant and forces which appear are compensated. Consequently, no oscillations are introduced into the pipeline system in which this mass flowmeter is installed, and oscillations of the pipeline system do not influence the measurement result.
In the case of mass flowmeters operating according to the Coriolis principle, which have only one straight Coriolis measuring tube, the positive consequence of Coriolis measuring tubes oscillating opposite one another, explained above, does not occur naturally. The center of mass does not remain constant and forces which appear are not compensated. The consequence of this is, on the one hand, that oscillations are transferred to the pipeline system in which a mass flowmeter is installed, and, on the other hand, that oscillations of the pipeline system can influence the measurement result.
In order to solve the above-mention problem, which is characteristic of mass flowmeters with one straight Coriolis tube operating according to the Coriolis principle, the pipeline system in which such a mass flowmeter is installed is frequently additionally clamped. As a rule, in this case, the tube guiding the flowing medium to the mass flowmeter and the tube guiding the flowing medium away from the mass flowmeter are clamped at a distance which corresponds to ten to fifteen times the diameter of the pipe.
In connection with the above-mentioned problem, which is characteristic of mass flowmeters with one straight Coriolis measuring tube operating according to the Coriolis principle, it has already been proposed to provide so-called anti-resonators where the Coriolis measuring tube is clamped, which anti-resonators should have a resonance spectrum of predetermined band width tuned to at least a characteristic oscillation of the Coriolis measuring tube (cf. European Offenlegungsschrift 0 521 439). However, it has been shown that such a measure on the whole cannot lead to an improvement in the case of mass flowmeters which operate very precisely in any event.
Initially, it was mentioned that the mass flowmeter on which the invention is based has a bridge located on the Coriolis measuring tube. Other names, namely "compensation cylinder", (in the German Patent Specification 41 24 295 and in German Offenlegungsschrift 41 43 361), or "supporting tube" (in German Patent Specification 42 24 379) are used for this bridge in the prior art. What is designated here as a bridge is what is designated otherwise as a "compensation cylinder" or "supporting tube". The more general expression "bridge" has been used here because in the case of this component, it does not have to be a matter of a cylinder or a tube. The interaction of Coriolis measuring tube and bridge is essential only to the effect that the axial separation of the Coriolis measuring tube/bridge connecting point specifies the functionally necessarily excited area of the Coriolis measuring tube, and the symmetrical arrangement and symmetrical design of the bridge with respect to the midpoint of the Coriolis measuring tube, wherein the midpoint of the measuring tube is defined with reference to the ends of the Coriolis measuring tube connected to the housing.
Also, it was mentioned initially that the mass flowmeter on which the invention is based has at least one oscillation generator acting on the Coriolis measuring tube and at least one sensor detecting Coriolis forces and/or Coriolis oscillations based on Coriolis forces and that the oscillation generator and the sensor "are operative between the Coriolis measuring tube and the bridge". This means that the Coriolis measuring tube is excited to oscillations with respect to the bridge and that Coriolis forces or Coriolis oscillations appearing between the Coriolis measuring tube and the bridge are detected by the sensor or, as a rule, by two sensors.